Transcoding of video signals

ABSTRACT

A method of transcoding from H.263 coded video frames to H.261 coded video frames, the method comprising for each INTER coded macro block (MB) of each frame, determining whether a component of the motion vector (MV) of the MB has a half pixel value, if yes, carrying out a MV search using the current H.263 decoded MB and a previous decoded frame to determine a new MV, the MV search being restricted to the vicinity of the H.263 coded MV, and coding the MB into an H.261 format using the new MV.

FIELD OF THE INVENTION

The present invention relates to the transcoding of video signals and inparticular to transcoding between H.263 and H.261 coded video signals.

BACKGROUND TO THE INVENTION

A number of video coding standards have recently been specified. A mainpurpose of these standards is to compress data to reduce the bandwidthrequired to transmit video signals over a communications link. State ofthe art ISDN video telephony terminals typically use the H.261 videocoding algorithm specified by the Technical division of theInternational Telecommunications Union (ITU-T), whilst terminals for usewith the proposed third generation 3GPP mobile networks will use theH.263 video coding algorithm specified by that same body. It will benecessary to ensure interworking of these two terminal types. Thecurrent proposals achieve interworking by providing in the communicationnetworks a transcoder which uses a “brute force” method to first of allcompletely decode a video signal coded using a first of the algorithms(H.263 or H.261), and then to recode the decoded signal using the secondof the algorithms. Transcoding between other video coding standards alsooften requires a similar brute force method. Transcoding in this wayrequires a great deal of processing power in the transcoder (where agreat many signals may require simultaneous transcoding) which is costlyin terms of network hardware. Transcoding is also time consuming,increasing transmission delay and degrading the perceived quality fordelay sensitive applications such as conversational video telephony.

STATE OF THE INVENTION

The present invention takes advantage of the common elements of videocoding algorithms to reduce the computational intensity of thetranscoding process. In particular, the need for a full motion vectorsearch is avoided during transcoding. According to a first aspect of thepresent invention there is provided a method of transcoding from H.263coded video frames to H.261 coded video frames both of which compriseVLC parameters, the method comprising for each INTER coded macro block(MB) of each frame:

-   -   determining whether a component of the motion vector (MV) of the        MB has a half pixel value, or if both components of the MV have        an even pixel value;    -   if both components have an even pixel value, performing said        transcoding by converting the H.263 VLC parameters into H.261        VLC parameters without performing a new MV search; and    -   if a component has a half pixel value, carrying out a MV search        using the current H.263 decoded MB and a previous decoded frame        to determine a new MV, the MV search being restricted to the        vicinity of the H.263 coded MV, and coding the MB into an H.261        format using the new MV.

It will be appreciated that embodiments of the present inventionsignificantly reduce the computational intensity of the transcodingprocess. The need for a MV search is either avoided completely (wherethe components of the H.263 coded MV are both integers) or the extent ofthe MV search is significantly reduced (from for example 32×32 MVcandidates to 2 or 4 MV candidates).

Preferably, said previous decoded frame is the previously coded H.261frame (i.e. that frame last processed by the transcoder) once moredecoded according to the H.261 standard in the transcoder.

Preferably, when a new MV search is carried out, said vicinity of the MVconsists of the two integer pixel locations on each side of the halfpixel component. Where both components of the H.263 coded MV have a halfpixel value, the vicinity consists of the four integer pixel locationssurrounding the MV.

Preferably, said step of coding the frame into an H.261 format using thenew MV comprises calculating a residual for each of the luminance andchrominance blocks by subtracting the corresponding blocks of theprevious decoded frame, shifted by the new MV or a value derived fromthat new MV, from the current H.263 decoded blocks. The residuals arethen DCT coded. The DCT coefficients are then quantised, zig-zagscanned, run length encoded, and VLC coded. Other parameters are alsoVLC coded. More preferably, in calculating the residuals for the colourblocks, the blocks of said previous decoded frame are shifted by amountswhich are the component values of the new MV divided by 2 and truncated(due to the fact that that the resolution of the colour blocks isdecimated by 2 in both dimensions).

Preferably, if both components of the H.263 coded MV of a MB areintegers, but at least one is odd, the residuals for the colour blocks(Cb and Cr) are recalculated using a MV which is the value of theoriginal MV divided by 2 and truncated. The MB is then coded bydetermining DCT coefficients for the new residuals. There is however noneed for a further MV search, nor is there a need to recalculateresiduals for the luminance blocks.

It is noted that if both MV components have an even pixel value, no MVsearch or recalculation of residuals or DCT coefficients is required.

For MBs for which recalculation of residuals is required because atleast one of the components of the H.263 coded MV has a half pixelvalue, loop filtering may be applied to the corresponding MV shiftedblock of the previous frame prior to subtracting it from the currentH.263 decoded block.

According to a second aspect of the present invention there is provideda method of transcoding between H.263 coded video frames and H.261 codedvideo frames, the method comprising for each INTER coded macro block(MB) of each frame:

-   -   determining whether or not the motion vector (MV) for that block        comprises a component which is an odd integer and, if so,        recalculating the residuals for at least the colour blocks of        the MB.

According to a third aspect of the present invention there is provided amethod of transcoding from H.261 coded video frames to H.263 coded videoframes, the method comprising for each INTER coded macro block (MB) ofeach frame:

-   -   determining whether or not loop filtering was used to generate        the H.261 coded MB;    -   if yes, calculating a new residual for each of the luminance and        colour blocks without applying a loop filter to the        corresponding decoded MB of the previous frame; and    -   coding the frame in an H.263 format.

The quality of the transcoding may be improved if the previous frame isthe H.263 coded and decoded frame, as compared to the decoded H.261coded frame.

Further if loop filtering was used and one or both of the H.261 coded MVcomponents is an odd integer, a bilinear interpolation may be used tohandle the half pixel resolution of H.263 coding for the colour blocks.

If loop filtering has not been used when H.261 coding a given MB, it maynot be necessary to recalculate residuals for the colour and luminanceblocks, compute DCT coefficients etc. However, a further check may becarried out to determine whether or not either of the H.261 coded MVcomponents is an odd integer. If so, residuals may be recalculated forthe colour blocks using the corresponding blocks of the previous frameshifted by half of the MV and using a bilinear interpolation to handlethe half pixel resolution used by H.263 coding.

Preferably, bilinear interpolation is used to handle the half pixelresolution of the H.263 coding.

According to a fourth aspect of the present invention there is provideda transcoder for providing an interface between an H.263 capable videotelephony terminal and an H.261 capable video telephony terminal, thetranscoder being arranged to implement one or more of the above first tothird aspects of the present invention.

According to a fifth aspect of the present invention there is provided amethod of transcoding from H.261 coded video frames to H.263 coded videoframes, the method comprising for each inter coded macro block (MB) ofeach frame:

-   -   conducting a MV search using a previous decoded frame, the        search being restricted to the 8 half pixel resolution shifts in        the vicinity of the H.261 full pixel resolution MV.

According to a sixth aspect of the present invention there is provided amethod of transcoding from H.261 coded video frames to H.263 coded videoframes, the method comprising for each inter coded macro block (MB) ofeach frame, coded using a loop filter on the originating H.261 side:

-   -   conducting a MV search using a previous decoded frame, the        search being restricted to the 8 half pixel resolution shifts in        the vicinity of the H.261 full pixel resolution MV.

According to a seventh aspect of the present invention there is provideda method of transcoding video frames from a first video coding format toa second video coding format, wherein both formats can represent a macroblock (MB) of a given frame using a motion vector (MV) defining adisplacement of the MB relative to a preceding frame, and wherein thefirst format specifies MV components to a resolution of a half pixel andthe second format specifies MV components to a resolution of one pixel,the method comprising for each INTER coded macro block (B) of eachframe:

-   -   determining whether a component of the motion vector (MV) of the        MB has a half pixel value, or if both components of the MV have        an even pixel value;    -   if both components have an even pixel value, performing said        transcoding by converting the first format VLC parameters into        second format VLC parameters without performing a new MV search;        and    -   if a component has a half pixel value, carrying out a MV search        using the current first format decoded MB and a previous decoded        frame to determine a new MV, the MV search being restricted to        the vicinity of the first format coded MV, and coding the MB        into the second format using the new MV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a system for communicating videosignals between an ISDN terminal and a 3GPP mobile terminal;

FIG. 2 is a flow diagram illustrating a method of transcoding from anH.263 coded video signal to an H.261 coded video signal; and

FIG. 3 is a flow diagram illustrating a method of transcoding from anH.261 coded video signal to an H.263 coded video signal.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

There is illustrated in FIG. 1 a system for communicating video signalsbetween an ISDN terminal 1 and a 3GPP mobile terminal 2. The ISDNterminal 1 is connected to a Public Switched Telephone Network (PSTN) 3whilst the mobile terminal 2 is connected to a 3GPP mobile network 4.The PSTN and 3GPP networks are coupled together via an interworking node5 which, whilst shown separately in FIG. 1, may be located in the PSTNor 3GPP networks or indeed in another network interconnecting the PSTNand 3GPP networks. One of the functions of the interworking node 5 is toperform transcoding of signals between the respective standards used bythe ISDN and 3GPP terminals.

As part of a video “conference” between the two terminals 1, 2, videosignals are exchanged. As already mentioned, the ISDN terminal 1 codesvideo signals generated at that terminal according to the H.261standard. The ISDN terminal 1 also expects to receive incoming videosignals coded using the same standard. On the other hand, the mobileterminal 2 codes video signals generated at that terminal according tothe H.263 standard and expects to receive incoming video signals codedusing the H.263 standard. Transcoding is performed at the interworkingnode 5 to overcome this mismatch of coding standards. For a detailedexplanation of the two coding standards, reference should be made to therelevant ITU-T documents. However, for the purpose of illustration, abrief explanation of each will now be provided.

H.263 Coding

The video signal provided to the coding unit at the 3GPP compatibleterminal 2 comprises a sequence of frames, each of which consists of anarray of pixels defined by red (R), green (G), and blue components (B).Taking each frame in turn, the coding unit sub-divides the frame into16×16 pixel macro blocks (MB). For each MB, the RGB pixel values areconverted into luminance (Y) values, blue difference chrominance values(Cb), and red difference chrominance values (Cr). The luminance valueshave the same spatial resolution as the RGB pixels, and are divided intofour 8×8 blocks per MB (referred to hereinafter as “luminance blocks”).The chrominance values have a spatial resolution which is half that ofthe RGB pixels in both dimensions, so that there is one 8×8 bluedifference chrominance block and one 8×8 red difference chrominanceblock per MB (referred to hereinafter as “colour blocks”). Coding iscarried out on a per MB basis, using the luminance and colour blocks ofthat MB.

A complete frame may be marked to be either INTRA or INTER coded by aparameter (PTYPE) in the header part of a frame. In addition, the headerof each MB of an INTER coded MB contains an indicator which marks anindividual MB as either INTRA or INTER coded.

Taking the first frame in a sequence, H.263 uses INTRA coding to codethe MBs of that frame. INTRA coding involves for each of the luminanceand colour blocks of a MB, determining a set of Discrete CosineTransform (DCT) coefficients which can be used to predict that block. Inorder to compress the block still further, the DCT coefficients arequantised, zig-zag scanned, run-length coded, and finally coded usingvariable length Huffman coding (VLC). The actual coded frame consists ofa frame header portion and a MB headers portion, followed by theindividual MB coded data. The header portions are typically VLC codedseparately from the MB data.

As the data present in any given frame is more often than not related tothe data present in the preceding frame, frames after the first frame ina sequence are coded using INTER coding. Following the generation of theluminance and colour blocks for each MB, a motion search is performedfor a MB by using the luminance pixels to find that shift which bestfits the MB to (a corresponding 16×16 pixel area of) the precedingframe. This shift is referred to as the “motion vector” (MV) for the MBand forms part of the compressed MB. The MV has x and y components andis determined to a resolution of one half pixel by using bilinearinterpolation to handle the full pixel resolution used for the luminancepixel array.

Once a MV has been determined for a MB, the residual is computed for theluminance blocks of the MB relative to the previous frame, shifted bythe MV. The process is repeated for the two colour blocks, except thatthat the MV is divided in two due to the lower resolution of the colourblocks. For each luminance and colour block, a DCT is applied to theresiduals. The DCT coefficients are quantised, zig-zag scanned andrun-length coded. Finally the coefficients and other data are codedusing Huffman VLC.

It is noted that where a MB contains no relevant information (e.g. allquantised DCT coefficients are equal to zero), the MB header willindicate this to avoid the need for subsequent coding and transmissionof the parameters of the MB. This MB may be skipped during the decodingprocess and the pixels of the MB are copied from the previous decodedframe based upon the assumption that nothing has changed in thecorresponding picture area

H.261 Coding

This form of coding is similar in principal to the H.263 codingdescribed above. The main differences are as follows:

-   -   The form of VLC used to code the DCT coefficient (and other        data) is different to that used by H.263.    -   The resolution with which H.261 coding calculates the MV is one        pixel and not one half pixel.    -   H.261 coding applies loop filtering (a bilinear low pass        filtering) to a decoded previous block shifted by the MV prior        to computing the residual for a block of the current flame. Both        unfilterd and filtered residuals for each block of a MB are        calculated, one of which is selected for further processing        according to which gives the lower signal power and consequently        can be coded with less bits. The choice is signalled further        towards the decoding side by a parameter (MTYPE) of a MB-header.

By taking into account these differences and, crucially, the points inthe coding process when they occur, it is possible to formulate atranscoding mechanism which significantly reduces the requirecomputational effort as compared to the brute force method.

H.263 to H.261 Transcoding

At the interworking node 5 of the system of FIG. 1, when a first, INTRAcoded frame is received, transcoding is a simple matter of convertingthe H.263 VLC form to the H.261 VLC form. This may be achieved byproviding a lookup table mapping the two different sets of VLC codes.Each INJRA coded MB, as well as each ENTER coded MB, is reconstructed atthe interworking node as part of the transcoding process. However,reconstruction (or decoding) is not computationally intensive whencompared to coding.

The interworking node 5 analyses the frame on a MB by MB basis. For eachINTER coded MB which does not contain only zero coefficients, the node 5examines the MV. If either of the x and y components have a half pixelvalue (e.g. 1.5, 2.5, etc), then a new MV search must be conducted. Thisrequires that the current H.263 decoded MB be compared against theprevious decoded frame. The new MV search is limited to the two fullpixel shifts on either side of the MV of the H.263 coding if only one ofthe components has a half pixel value, and to the four full pixel shiftssurrounding the MV if both components have a half pixel value. Thus forexample if the H.263 coded MV is (7, 6.5) then the search will examinethe candidate MVs (7, 6) and (7, 7) to find the best fit. If the H.263coded MV is (4.5, −2.5) then the search will examine the candidate MVs(4, −3), (4, −2), (5, −3), and (5, −2).

Using the new MV, residuals are computed for each of the luminance andcolour blocks, based on the decoded form of the previous H.261 codedframe (this gives better results than using the decoded previous H.263frame directly). Respective sets of DCT coefficients calculated (NB.calculation of the residuals for the colour blocks uses a modified formof the MV—see below). The quantised, zig-zag scanned and run-lengthcoded DCT coefficients and other data are VLC coded into H.261 form.

In the event that the examination of a MV for a MB determines thatneither of the MV components has a half pixel value, it is thendetermined whether or not either has a value which is an odd integer.During the H.261 decoding, the colour blocks of a MB of the previousframe must be shifted relative to the corresponding colour blocks of thecurrent frame using the MV. However, because the resolution of thecolour blocks is half that of the luminance block (and hence half thatof the MV), the MV components must be divided in two to determine theshift of the colour blocks. The H.261 decoding rounds the divided MVcomponents towards zero (i.e. negative values upwards and positivevalues downwards), and shifts the colour blocks by the rounded values(i.e. MV_(colour)=TRUNC(MV/2). H.263 decoding on the other hand uses thedivided vales directly, even if these involve a half pixel componentbecause half pixel resolution for the MVs can be handled in H.263 withthe help of bilinear interpolation. H.261 coding takes account of therounding of MV components in the residuals for the colour blocks—H.263does not. Therefore, if one or both of the components are odd integers,the residuals are recalculated for the colour blocks (using the dividedand rounded MV). It will be appreciated that this process does nothowever require recalculation of the MV, nor does it requirerecalculation of the residuals for the luminance block of the MB). It isnoted that recalculation of the residuals for the colour blocks, takingaccount of the divided and rounded MV, is carried out automatically ifone or both of the MV components are determined to have a half pixelvalue.

In summary, for H.263 to H.261 transcoding of an INTER coded MB, if bothof the MV components are even integers, only mapping of the VLC codedDCT coefficients is required. If one or both of the MV components havehalf pixel values, a limited MV search must be carried out, followed byrecalculation of the residuals and DCT coefficients for the luminanceand colour blocks. The results are coded using the appropriate VLC form.If one of the MV components is an odd integer, and the other does nothave a half pixel value, no further MV search is required—only theresiduals for the colour blocks need to be recalculated.

When at least one of the components of an H.263 coded MV is not aninteger, a loop filter (as specified for H.261) may or may not beapplied to the corresponding predicted MB of the previous frame.However, application of a loop filter will typically result in, animprovement in the perceived quality of the displayed video although itdoes require more processing capacity.

H.261 to H.263 Transcoding

Transcoding of INTRA coded frames or MBs again requires only atransformation of the VLC coding. Two checks are performed for each ORcoded MB. If the answers to both of these checks are no, then onlytransformation of the VLC coding is required.

The first check is to see if the H.263 coding applied a loop filter tothe previous decoded and MV shifted block when coding the present MB.This can be determined from a flag in the MB header. If the answer isyes, then the residuals for the luminance and chrominance blocks of thecurrent MB are recalculated, this time without applying the loop filterto the previous decoded and MV shifted block. The residuals are DCTcoded, quantised, zig-zag scanned, run-length coded, and finally VLCcoded. No further MV search is required.

The second check is to see if either of the MV components is an oddinteger. If so, then the H.261 coded residuals for the colour blockswill have been calculated based on the previous corresponding colourblocks shifted by the TRUNC(MV/2) as described above. The H.263residuals for the colour blocks must be recalculated, using the halfpixel MV/2 together with the bilinear interpolation method defined forH.263 coding. Again, no further MV search is required, and indeed it isnot necessary to recalculate the residual for the luminance block. Thissecond check is only required if the answer to the first check was no,as if loop filtering was used, recalculation of the colour blocks willautomatically use a half pixel shift of MV/2 where appropriate, and thenecessary bilinear interpolation.

A further enhancement to the perceived quality of the transcoded signal(H.261 to H.263) may be achieved if a mini search for a half pixelresolution H.263 MV is made on the eight half pixel positions in theimmediate vicinity of the full pixel H.261 MV. For example, in thevicinity of a full pixel MV (3, 4) there are the following eight halfpixel MV candidates: (2.5, 3.5), (2.5, 4), (2.5, 4.5), (3, 3.5), (3,4.5), (3.5, 3.5), (3.5, 4) and (3.5, 4.5). This requires more processingcapacity than above, but still less than the brute force transcodingapproach. The mini search can be made for every INTER coded MB or, inorder to save the average processing power, only for those MBs whichindicate that loop filtering has been used on the H.261 side.Recomputing and coding of residual is of course required if a moreoptimal half pixel MV is found in the mini search

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiment withoutdeparting from the scope of the present invention. For example, inaddition to video conferencing, the present invention may be applied forexample to streaming video transmission. It will also be appreciatedthat various transformations for parsing between different embodimentsof the H.261 and H.263 standards may be necessary. However, these willbe readily apparent to the skilled person. It will also be appreciatedthat the present invention may be applied to other transcodingprocesses, for example transcoding between H.263 and MPEG.

1. A method of transcoding from H.263 coded video frames to H.261 codedvideo frames both of which comprise VLC parameters, the methodcomprising for each INTER coded macro block (MB) of each frame:determining whether a component of the motion vector (MV) of the MB hasa half pixel value; if yes, carrying out a MV search using the currentH.263 decoded MB and a previous decoded frame to determine a new MV, theMV search being restricted to the vicinity of the H.263 coded MV; andcoding the MB into an H.261 format using the new MV.
 2. The methodaccording to claim 1, wherein said previous decoded frame is thepreviously decoded H.261 frame within the transcoder.
 3. The methodaccording to claim 2, wherein, when a new MV search is carried out, saidvicinity of the MV consists of the two integer pixel locations on eachside of the half pixel component where one of the MV components has ahalf pixel value, and consists of the four integer pixel locationssurrounding the MV where both of the MV components have half pixelvalues.
 4. The method according to claim 3, wherein said step of codingthe frame into an H.261 format using the new MV comprises calculating aresidual for each luminance and colour block by subtractingcorresponding blocks of a previous decoded frame, shifted by the new MVor a value derived from the new MV, from the current H.263 decodedblocks.
 5. The method according to claim 4, wherein, when calculatingthe residuals for the luminance and colour blocks, the blocks areshifted by an amount which is the truncation of the new MV divided by 2.6. The method according to claim 5, wherein, if a component of the MVfor a given MB has an odd integer value, and the other component doesnot have a half pixel value, the residuals for the luminance and colourblocks are recalculated using a MV which is the value of the original MVdivided by 2 and rounded towards zero.
 7. The method according to claim6, wherein, for MBs which have a MV having a half pixel component, loopfiltering is applied to the corresponding decoded MV-shifted block ofthe previous frame when recalculating residuals.
 8. A method oftranscoding between H.263 coded video frames and H.261 coded videoframes, the method comprising for each INTER coded macro block (MB) ofeach frame: determining whether the motion vector (MV) for that blockcomprises a component which is an odd integer and, if so, recalculatingthe residuals for at least each of the colour blocks of the MB.
 9. Amethod of transcoding from H.261 coded video frames to H.263 coded videoframes, the method comprising for each INTER coded macro block (MB) ofeach frame: determining whether loop filtering was used to generate theH.261 coded MB; if yes, calculating a new residual for each luminanceand colour block without applying a loop filter to the correspondingdecoded MV shifted block of the previous frame; and coding the frame inan H.263 format.
 10. The method according to claim 9, wherein, if loopfiltering has not been used when H.261 coding a given MB, a furthercheck is carried out to determine whether or not either of the MVcomponents is an odd integer and, if so, residuals are recalculated forthe luminance and colour blocks using blocks of the corresponding MB ofthe previous frame shifted by half of the MV.
 11. The method accordingto claim 10 and comprising using bilinear interpolation to handle halfpixel MV component resolutions when recalculating residuals for colourblocks.
 12. (canceled)
 13. A method of transcoding from H.261 codedvideo frames to H.263 coded video frames, the method comprising for eachinter coded macro block (MB) of each frame: conducting a MV search usinga previous decoded frame, the search being restricted to the 8 halfpixel resolution shifts in the vicinity of the H.261 full pixelresolution MV.
 14. A method of transcoding from H.261 coded video framesto H.263 coded video frames, the method comprising for each inter codedmacro block (MB) of each frame, coded using a loop filter on theoriginating H.261 side: conducting a MV search using a previous decodedframe, the search being restricted to the 8 half pixel resolution shiftsin the vicinity of the H.261 full pixel resolution MV.
 15. A transcoderfor providing an interface between an H.263 capable video telephonyterminal and an H.261 capable video telephony terminal, the transcoderbeing arranged to implement the method of claim
 14. 16. A method oftranscoding video frames from a first video coding format to a secondvideo coding format, wherein both formats can represent a macro block(MB) of a given frame using a motion vector (MV) defining a displacementof the MB relative to a preceding frame, and wherein the first formatspecifies MV components to a resolution of a half pixel and the secondformat specifies MV components to a resolution of one pixel, the methodcomprising for each INTER coded macro block (MB) of each frame:determining whether a component of the motion vector (MV) of the MB hasa half pixel value; if yes, carrying out a MV search using the currentdecoded MB and a previous decoded frame to determine a new MV, the MVsearch being restricted to the vicinity of the original MV; and codingthe MB into said second format using the new MV.
 17. The methodaccording to claim 1, wherein, if neither of the MV components has ahalf pixel value or is an odd integer, the VLC coded data is transcodedfrom the H.263 form to the H.261 form.
 18. A transcoder for providing aninterface between an H.263 capable video telephony terminal and an H.261capable video telephony terminal, the transcoder being arranged toimplement the method of claim
 7. 19. A transcoder for providing aninterface between an H.263 capable video telephony terminal and an H.261capable video telephony terminal, the transcoder being arranged toimplement the method of claim
 8. 20. A transcoder for providing aninterface between an H.263 capable video telephony terminal and an H.261capable video telephony terminal, the transcoder being arranged toimplement the method of claim
 11. 21. A transcoder for providing aninterface between an H.263 capable video telephony terminal and an H.261capable video telephony terminal, the transcoder being arranged toimplement the method of claim
 13. 22. A transcoder for providing aninterface between an H.263 capable video telephony terminal and an H.261capable video telephony terminal, the transcoder being arranged toimplement the method of claim
 14. 23. A transcoder for providing aninterface between an H.263 capable video telephony terminal and an H.261capable video telephony terminal, the transcoder being arranged toimplement the method of claim 16.